Technical Field
The present invention relates to a switching controller that implements frequency spreading control for a switching power supply.
Background Art
Typically, switching power supplies turn a switching element ON/OFF using pulse signals to generate a desired output voltage Vout from an input voltage Vin. These switching power supplies also typically include a switching controller that keeps the output voltage constant even if the input voltage or the load current fluctuates. Such switching controllers typically utilize a pulse-width modulation (PWM) control scheme in which the output voltage error (that is, an amplified signal of the difference between the output voltage and a target voltage) is detected and the ratio of time the pulse signal is ON to OFF (that is, the duty cycle) is adjusted accordingly.
In recent years, regulations on electromagnetic interference (EMI) generated by electronic devices have been established. In switching power supplies, harmonic components in the frequencies of the pulse signals that control the switching element (hereinafter, “switching frequency fsw”) can pose a problem. One known solution to this problem is to include a frequency sweeper in the switching controller of the switching power supply and modulate the switching frequency fsw to a desired shift amount Δfsw in order to reduce the average value in the noise spectrum (see Patent Documents 1 and 2, for example).
Meanwhile, in order to improve power conversion efficiency as well as reduce switching loss (which becomes the dominant form of loss during light loading), switching power supplies also sometimes utilize a frequency control scheme in which the switching frequency is decreased as the load current decreases (that is, a pulse frequency modulation (PFM) control scheme) in addition to the PWM control scheme.